Axial piston pumps and motors are used in myriad systems and environments. Axial piston pumps and motors generally include a housing, a rotor, a port plate, a hanger (or swash plate), and a stack-up assembly. The rotor is rotationally mounted within the housing, and has a number of piston bores formed therein. A piston is movably inserted into each one of the piston bores. The port plate is non-rotationally mounted within the housing adjacent one end of the rotor, and includes a low-pressure side and a high-pressure side. The hanger is also non-rotationally mounted in the housing but may be allowed to pivot about a central axis ninety degrees from the rotor axis. The hanger is disposed at an opposite end of the rotor and at an angle relative to the rotational axis of the rotor. The stack-up assembly is coupled to the angularly disposed hanger and to each of the pistons, and typically includes a cam plate, an auxiliary cam, and an auxiliary cam retainer. During operation, the pistons are cyclically pushed into and/or pulled from the piston bores, depending upon whether the machine is implemented as a pump or a motor.
The pistons in axial piston pumps and motors are typically coupled to piston shoes, which are in turn typically coupled to the stack-up assembly. The piston shoes slidingly engage the cam plate at a piston shoe-to-cam plate interface. The piston shoes may be crimped onto rounded heads of the pistons to form a piston-to-shoe interface. Although the materials that comprise the pistons and piston shoes are selected and processed to achieve wear resistance, lubrication may still be needed. As such, each piston may include an internal channel that extends through it to a feed port at the apex of the rounded head. The primary purpose of this channel is to provide lubrication, via a passageway formed through the piston shoe, to the piston shoe-to-cam plate interface.
Some of the liquid that flows through the internal channel in the piston is also preferably used to lubricate the piston-to-shoe interface. However, test data show that under certain high-load conditions there may be insufficient lubrication at the piston-to-shoe interface. This lack of sufficient lubrication is most evident at the portion of the piston shoe that is furthest from the feed port at the apex of the rounded head. When operating as a pump, the piston-to-shoe interface contact loads are the highest at this portion of the shoe during the intake portion of the operational cycle. At this point in the cycle, the pressure of the liquid being supplied to the feed port is also at a minimum, and may even be less than pump case pressure. Thus, there may be little or no driving force, other than capillary action, to drive lubricant into the piston-to-shoe interface at this point of the cycle, resulting in wear of the piston and/or piston shoe. The resulting wear can lead to increased axial endplay at the piston-to-shoe interface.
Hence, there is a need for a method of providing and maintaining lubrication at a piston-to-shoe interface in axial piston pumps and motors. The present invention addresses at least this need.